In the state of the art, different methods are known which lead to an increase in corrosion resistance of articles having a chromium coating as decorative finish. Such items can be plastic parts, brass articles, aluminum alloys and zinc die cast parts or also steel bodies. These parts having a chromium coating are applied in many areas, in particular in sanitary facilities, automotive and aerospace.
Electrolytic chromium and nickel depositions are generally chosen for realizing of a high corrosion resistance. In this regard, the nickel layer is divided in three different types. The first type is known as semi-bright nickel layer or sulfur-free layer, because it is a semi-bright layer having a sulfur content <0.005 weight-%. These layers have a higher electrochemical potential than bright nickel layers.
On top of the semi-bright nickel layer, a bright nickel layer is regularly electroplated. This leads to a bright appearance of the coated articles. These layers have a sulfur content of more than 0.03 weight-%.
The last nickel layer is a layer which has small disruptions on a micro-scale. This layer can comprise micro-particles or organic additives and can be coated with a chromium layer which has a micro-porous layer or a layer with micro cracks. These layers are usually nobler than bright nickel layers i.e. their potential is higher than that of bright nickel layers. Such coatings are known from U.S. Pat. No. 3,268,424 and U.S. Pat. No. 3,563,864. In these applications, the main aim is to decrease galvanic corrosion between chromium and nickel. The chromium layer is thereby electroplated as finish with an electrolyte comprising hexavalent chromium.
A further process is known which increases corrosion protection of the above-mentioned parts described before which are electroplated. In this regard, EP 1 343 924 B1 discloses a layer of silver or silver alloy which is located between the chromium and nickel layer. It is a problem that very toxic cyanide salts are used in the process which represent a serious threat for health and safety and are therefore no longer acceptable with regard to environmental aspects. Furthermore, silver as noble metal demonstrates two important disadvantages which are the high cost as well as the significant difference of the electrochemical potential in comparison to a bright nickel layer.
Different electrolytes based on trivalent chromium have been developed for the deposition of chromium layers over the years to prevent the use of environmentally precarious hexavalent chromium. Such processes are disclosed in EP 0 058 044 and GB 1 455 580. Trivalent chromium electrolytes have been used for years as decorative coatings, but show the disadvantage that they do not demonstrate sufficient corrosion resistance because it is not a pure chromium layer, but a special alloy comprising constituents of chromium, carbon, iron, sulfur, oxygen and nitrogen and thus have structural features different to pure chromium. Commonly, the UNI EN ISO 9227 CASS standard procedure (so-called CASS test) is applied for the investigation of the corrosion resistance of coated parts. In this test, the corrosion resistance (in hours) is measured in a room filled with salt spray at 50° C., wherein the salt consists of a sodium chloride solution which comprises copper ions with acetic acid (pH 3).
In recent years, a new test procedure has been introduced in the automotive industry to solve the problem that calcium chloride is used as antifreeze on frozen streets in northern countries. It turned out that calcium chloride reacts very aggressively with chromium covered parts. This is the reason why identical tests were introduced by e.g. Volkswagen (VW PV1067) and Nissan (NES M4063) (so-called “Russian Mud test”), in which the resistance of chrome deposited parts can be determined by using calcium chloride in the corrosion test.